<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<button id="btn1">按钮1</button>
<button id="btn2">按钮2</button>
<button onclick="demo()">按钮3</button>

<script type="text/javascript">
    const btn1 = document.getElementById('btn1')
    btn1.addEventListener('click', () => {
        alert('按钮1被点击了')
    })

    const btn2 = document.getElementById('btn2')
    btn2.onclick = () => {
        alert('按钮2被点击了')
    }

    function demo() {
        alert('按钮3被点击了')
    }


</script>
</body>
</html>